1. Field of the Invention
The present invention relates to monomers suitable for the preparation of PNA oligomers. The present invention also relates to precursors to the monomers and methods of making the PNA monomers from the precursors. Further, the invention relates to methods of making PNA oligomers using the PNA monomers.
2. General Background and State of the Art
In the last two decades, attempts to optimize the properties of oligonucleotide by modification of the phosphate group, the ribose ring, or the nucleobase have resulted in a lot of discoveries of new oligonucleotide derivatives for the application in the fields of DNA diagnostics, therapeutics in the form of antisense and antigene, and the basic research of molecular biology and biotechnology (U. Englisch and D. H. Gauss, Angew. Chem. Int. Ed. Engl. 1991, 30, 613–629; A. D. Mesmaeker et al. Curr. Opinion Struct. Biol. 1995, 5, 343–355; P. E. Nielsen, Curr. Opin. Biotech., 2001, 12, 16–20.). The most remarkable discovery is peptide nucleic acid which was reported by the Danish group of Nielsen, Egholm, Buchardt, and Berg (P. E. Nielsen et al., Science, 1991, 254, 1497–1500). PNA is DNA analogue in which an N-(2-aminoethyl)glycine polyamide replaces the phosphate-ribose ring backbone, and methylene-carbonyl linker connects natural as well as unnatural nucleo-bases to central amine of N-(2-aminoethyl)glycine. Despite radical change to the natural structure, PNA is capable of sequence specific binding to DNA as well as RNA obeying the Watson-Crick base pairing rule. PNAs bind with higher affinity to complementary nucleic acids than their natural counterparts, partly due to the lack of negative charge on backbone, a consequently reduced charge-charge repulsion, and favorable geometrical factors (S. K. Kim et al., J. Am. Chem. Soc., 1993, 115, 6477–6481; B. Hyrup et al., J. Am. Chem. Soc., 1994, 116, 7964–7970; M. Egholm et al., Nature, 1993, 365, 566–568; K. L. Dueholm et al., New J. Chem., 1997, 21, 19–31; P. Wittung et al., J. Am. Chem. Soc., 1996, 118, 7049–7054; M. Leijon et al., Biochemistry, 1994, 9820–9825.). Also it was demonstrated that the thermal stability of the resulting PNA/DNA duplex is independent of the salt concentration in the hybridization solution (H. Orum et al., BioTechniques, 1995, 19, 472–480; S. Tomac et al., J. Am. Chem. Soc., 1996, 118, 5544–5552.). And PNAs can bind in either parallel or antiparallel fashion, with antiparallel mode being preferred (E. Uhlman et al., Angew. Chem. Int. Ed. Engl., 1996, 35, 2632–2635.).
A mismatch in a PNA/DNA duplex is much more destabilizing than a mismatch in a DNA/DNA duplex. A single base mismatch results in 15° C. and 11° C. lowering of the Tm of PNA/DNA and DNA/DNA, respectively. Homopyrimidine PNA oligomers and PNA oligomers with a high pyrimidine/purine ratio can bind to complementary DNA forming unusually stable PNA2/DNA triple helices (P. E. Nielsen et al., Science, 1991, 254, 1497–1500; L. Betts et al. Science, 1995, 270, 1838–1841; H. Knudsen et al., Nucleic Acids Res., 1996, 24, 494–500.). Although PNAs have amide bonds and nucleobases, PNAs show great resistance to both nuclease and protease. In contrast to DNA, which depurinates on treatment with strong acids and hydrolyses in alkali hydroxides, PNAs are completely acid stable and sufficiently stable to weak bases.
Generally, PNA oligomers are synthesized using the well established solid phase peptide synthesis protocol. New strategies for monomers have been developed independently by several groups to optimize PNA oligomer synthesis. The preparation of PNA monomers can be divided into the synthesis of a suitably protected N-aminoethylglycine and a suitably protected nucleobase acetic acid derivatives, which is followed by coupling both.
The first synthetic strategy reported for PNA oligomer synthesis was Merrifield solid phase synthesis using t-Boc/benzyloxycarbonyl protecting group strategy wherein the backbone amino group protected with the t-Boc and the exocyclic amino groups of the nucleobases are protected with the benzyloxycarbonyl (P. E. Nielsen et al., Science, 1991, 254, 1497–1500; M. Egholm et al., J. Am. Chem. Soc., 1992, 114, 9677–9678; M. Egholm et al., J. Am. Chem. Soc., 1992, 114, 1895–1897; M. Egholm et al., J. Chem. Soc. Chem. Commun., 1993, 800–801; K. L. Dueholm et al., J. Org. Chem., 1994, 59, 5767–5773; WO 92/20702). PNA monomers protected with t-Boc/benzyloxycarbonyl are now commercially available but are inconvenient to use because repeated treatment with TFA is required for t-Boc deprotection and the harsh HF or trifluoromethanesulfonic acid treatment required for cleavage from the resin and deprotection of benzyloxycarbonyl group from exocyclic amine of nucleobases. Thus, this strategy is not compatible with the synthesis of many types of modified PNA oligomers such as PNA-DNA chimera. Furthermore, the use of hazardous acids, such as HF or trifluoromethanesulfonic acid, is not commercially embraced in view of safety concerns for the operator and the corrosive effect on automation equipment and lines. In addition, the t-Boc/benzyloxycarbonyl protection strategy is differential strategy which is defined as a system of protecting groups wherein the protecting groups are removed by the same type of reagent or condition, but rely on the different relative rates of reaction to remove one group over the other. For example, in the t-Boc/benzyloxycarbonyl protecting strategy, both protecting groups are acid labile, but benzyloxycarbonyl group requires a stronger acid for efficient removal. When acid is used to completely remove the more acid labile t-Boc group, there is a potential that a percentage of benzyloxycarbonyl group will also be removed contemporaneously. Unfortunately, the t-Boc group must be removed from amino group of backbone during each synthetic cycle for the synthesis of oligomer. Thus TFA is strong enough to prematurely deprotect a percentage of the side chain benzyloxycarbonyl group, thereby introducing the possibility of oligomer branching and reducing the overall yield of desired product.
In another effort to find a milder deprotecting method for PNA oligomer synthesis that would be compatible with DNA oligomer synthesis, several research groups have developed PNA monomers protected with Mmt/acyl wherein the backbone amino group protected with the Mmt and the exocyclic amino groups of the nucleobases are protected with an acyl group such as benzoyl, anisoyl, and t-butyl benzoyl for cytosine and adenine, or isobutyryl, acetyl for guanine (D. W. Will et al., Tetrahedron, 1995, 51, 12069–12082; P. J. Finn et al., Nucleic Acid Research, 1996, 24, 3357–3363; D. A. Stetsenko et al., Tetrahedron Lett. 1996, 3571–3574; G. Breipohl et al., Tetrahedron, 1997, 14671–14686.).
Alternative PNA monomers protected with Fmoc/benzhydryloxycarbonyl are also commercially available wherein the backbone amino group protected with the Fmoc and the exocyclic amino groups of the nucleobases are protected with the benzhydryloxycarbonyl (J. M. Coull, et al., U.S. Pat. No. 6,133,444). But Fmoc/benzhydryloxycarbonyl strategy has several drawbacks such as side reaction during the Fmoc deprotection process and instability of monomer in solution. The most critical side reaction is the migration of the nucleobase acetyl group from the secondary amino function to the free N-terminal amino function of aminoethylglycine backbone under Fmoc deprotection condition (L. Christensen et al., J. Pept. Sci. 1995, 1,175–183). The-N-acetyl transfer reactions in every cycles during oligomer synthesis result in accumulation of side products which are hard to separate due to similar polarity and same molecular weight. Also the Fmoc protecting group is very unstable in the presence of trace amine. Thus, the selection of the solvent for the PNA monomers should be cautious. Generally, N-methylpyrrolidone of high quality is recommended. This requires higher cost in the synthesis of PNA oligomer.
The synthesis of PNA oligomers using Fmoc/benzyloxycarbonyl (S. A. Thomson et al., Tetrahedron, 1995, 6179–6194.) and Fmoc/Mmt (G. Breipohl et al., Bioorg. Med. Chem. Lett., 1996, 6, 665–670.) protected monomer has also been reported. However, all of these methods have serious drawbacks in terms of monomer solubility and preparation, harsh reaction condition, and side reactions either during monomer synthesis and/or PNA oligomer synthesis.
In other efforts to find new monomers, cyclic monomers were reported by ISIS and Biocept. The first strategy developed by ISIS replaces protected backbone by morpholinone (U.S. Pat. No. 5,539,083), but the strategy has serious drawback in that the hydroxy functional group generated by coupling reaction should be converted to amine functional group in every elongation step during oligomer synthesis. Alternatively, the protected aminoethylglycine part is replaced by N-t-Boc-piperazinone (WO 00/02899). However, this strategy also has several drawbacks in terms of monomer reactivity in oligomerization and the same problems as seen in linear t-Boc strategy as described above.
Despite recent advances, there remains a need for new monomer that increases yield, lowers synthetic cost, and is suitable for automatic and parallel synthesis.